Previous   |    Next

doi:10.2204/iodp.proc.327.103.2011

Hydrologic experiments

Hole U1362A

In Hole U1362A the single-element drill string packer was successfully inflated in open hole to assess the transmissivity and average permeability of the isolated zone from the packer seat at 426.5 mbsf (3098.5 mbrf) to the bottom of the hole. The packer seat was chosen primarily on the basis of caliper information from the wireline log and was also intended to be the seat for the CORK open hole packers. A depth check with the packer BHA just before packer inflation found the hole open to 519 mbsf (9 m shallower than the original drilled depth). Thus, the bottom of the test interval is uncertain and estimated to be 519–528 mbsf.

A subsequent attempt to inflate the packer in casing to estimate the transmissivity of the entire open hole interval failed because of damage to the inflation element, which was evident upon recovery of the BHA and packer. This damage may have occurred during the depth check prior to the open hole inflation or during movement of the packer following the first set of tests.

The ship’s mud pump was used for all packer operations. Two Micro-Smart electronic pressure gauges (SN 40060 at 5 s sample interval and SN 4986 at 10 s sample interval) were deployed in the packer go-devil, and both worked well. The record of gauge pressure and internal temperature for the SN 40060 is shown in Figure F46. The packer seat was originally intended to be at 424.5 mbsf. After the go-devil landed at the intended inflation depth but before the packer was inflated, hydrostatic baseline pressure was recorded for 1 h. It was difficult to inflate the packer at the intended depth, so the packer was moved 2 m deeper, where it was successfully inflated at ~1000 psi (6.9 MPa). After inflation, the sealed-hole pressure baseline was recorded for 1 h, and then two constant-rate injection tests were conducted. Under the expectation that permeabilities in the deep basement section in Hole U1362A would be comparable to those determined during Expedition 301, deep in Hole U1301B (Becker and Fisher, 2008), similar inflation rates were used: 13 and 25 spm, or 4.4 and 8.4 L/s, respectively. Injection periods were 1 h long, and each was followed by a 1 h period of pressure recovery.

The offset between the initial hydrostatic and borehole pressure readings was expected from the effects of pumping cold, dense seawater during drilling operations, inducing a near-borehole apparent underpressure even though the natural formation state is overpressured. During both injection periods, downhole pressures rose in a manner consistent with the predicted response, indicating a good packer seal. Initial shipboard processing of the data from the first injection period shows a good fit to the expected linear rise of pressure versus natural log of time predicted by the Theis (1935) formulation for an injection test. The slope of the log-linear response indicates a bulk permeability for the isolated zone of ~2 × 10^–12 m², the same as the average value reported by Becker and Fisher (2008) for a comparable section of basement in nearby Hole U1301B. This preliminary result needs to be refined with more sophisticated postcruise processing. Agreement with the value from Hole U1301B is not surprising given the similarity of the logs from the two holes. If confirmed, it would be the first direct indication of consistency of a layered hydrological structure in upper oceanic crust over lateral scales of several hundred meters.

Hole U1362B

Before installing the CORK in Hole U1362B, we ran a 24 h pumping and tracer injection experiment using a 10¾ inch casing running tool in lieu of a drill string packer to seal the hole. The configuration of shipboard systems and the preparation of tracers for this experiment are discussed in detail in Fisher, Cowen, et al., but a summary of operations and results is provided herein.

A depth check following hole conditioning and prior to the start of the pumping experiment indicated that there was open hole to 353.0 mbsf (111.0 msb), meaning that the pumping test would sample conditions through 81 m of upper basement. The casing running tool was made up above a stinger comprising 3 drill collars, 25 joints of drill pipe, and a specially constructed sub containing fast-sampling OsmoSamplers and pressure gauges (the same Micro-Smart tools used for packer testing). The stinger was designed and spaced out to place the pressure gauges within open hole just below the shoe of the 10¾ inch casing. The OsmoSamplers were placed above the pressure gauges but were separated from them by a solid disk of steel. The fluid samplers were housed in a chamber that connected the drill pipe above to slots in the wall of the chamber. With this design, fluids pumped down the drill pipe would wash past the fluid samplers before being jetted out into the hole, where pressure was measured.

The first attempt at running the 24 h injection experiment failed when the stinger encountered fill in the hole prior to landing the casing running tool (see “Operations”). After the hole was cleaned out, a reconfigured (shortened) stinger was deployed with reset OsmoSamplers and pressure gauges. We ran a circulation test with the rig pumps before reentering the hole to determine the expected response on rig gauges to various pumping rates with the mud pumps. With this test complete, the hole was reentered and the 10¾ inch casing hanger was landed in the throat of the reentry cone, creating a hydrologic seal between the ocean above and the open hole below. We waited 1 h to establish a formation baseline and then turned on the mud pumps at 20 spm, equivalent to ~6.7 L/s, approximately the same rate of fluid flow as that inferred in Hole U1301B after Expedition 301, leading to a significant pressure perturbation in Hole 1027C, 2.4 km to the north-northeast (Fisher et al., 2008). Immediately after the start of the 24 h pumping experiment, we began injecting seawater saturated with dissolved SF₆ using a manifold and automated switching valves (Fisher, Cowen, et al.).

The primary injectate for the Expedition 327 experiment was seawater, but after 2 h of pumping we injected freshwater (“drill water”) for 1 h, and then again 17 h later. In addition, we had four brief periods during which we added salt tracers in solution to the injectate using the cement pump system: CsCl and ErCl₃ (salt injection 1) dissolved in seawater, CsCl and HoCl₃ (salt injection 2) dissolved in seawater, fluorescent microspheres in freshwater, and fluorescent-stained bacteria filtered from surface seawater and injected with seawater.

The test continued as planned for 24 h, and then the mud pump was stopped. Conditions were monitored for 1 h without moving the casing running tool to establish a final pressure baseline. The casing running tool was lifted from the casing hanger while being monitored with the VIT camera to confirm that there was no flow out of the casing, and then the pipe was retrieved to recover the fluid sampler and pressure gauges in the stinger. OsmoSampler sample coils (two of which were PTFE and two of which were copper) were extracted from the stinger and will be analyzed after the cruise to determine the composition of the injectate as it left the stinger. The pressure gauges provided a high-quality record of conditions in the hole during the experiment (Fig. F47).

Pressure measured at the stinger dropped by ~1.5–2 psi immediately after the casing running tool was landed in the hanger because there was a small negative pressure in the borehole created by the rapid flow of cold bottom water down the hole and into the formation. Pressure subsequently rose when pumping began, and the record became irregular when the primary injectate was switched from saltwater to freshwater. Pressure rose after the switch back to saltwater, probably coinciding with freshwater flowing out of the stinger and down the borehole. The local tidal cycle is clearly expressed during the middle two-thirds of the record, and the four periods of solute and suspended tracer injection are readily apparent as short (8–9 min) intervals of elevated pressure. The second period of freshwater injection was followed by another 1 h period of elevated pressure, again most likely caused by the pressure difference created as freshwater flowed down the borehole. The excess pressure created by pumping dropped immediately after pumping ceased, and a formation pressure baseline was reestablished. The relatively small pressure rise associated with the long period of pumping is consistent with the formation being highly permeable, but careful analysis will be required to separate the various signals that are convolved in the pressure record from this experiment.

Top of page   |    Previous   |    Next